Since energy issues and global environmental issues are becoming more serious, solar cells are receiving more attention as an alternative energy for replacing fossil fuels. In the solar cell, carriers (electrons and holes) generated by light irradiation to a photoelectric conversion section composed of a semiconductor junction or the like are extracted to an external circuit to generate electricity. A collecting electrode is provided on the photoelectric conversion section of the solar cell for efficiently extracting carriers generated at the photoelectric conversion section to the external circuit.
For example, in a crystalline silicon-based solar cell using a single-crystal silicon substrate or a polycrystalline silicon substrate, a collecting electrode made of a slender metal is provided on a light receiving surface. Also, in a heterojunction solar cell having amorphous silicon layers and transparent electrode layers on a crystalline silicon substrate, collecting electrode(s) are provided on the transparent electrode layer(s).
In a silicon-based thin-film solar cell using an amorphous silicon thin-film, a crystalline silicon thin-film or the like, and a thin-film solar cell such as a compound solar using CIGS, CIS or the like, an organic thin-film solar cell or a dye-sensitized solar cell, a transparent electrode layer is provided on a surface of a photoelectric conversion section on the light receiving surface side in order to reduce the surface resistance of the light receiving surface. In this configuration, the transparent electrode layer can perform a function as a collecting electrode, and therefore in principle, it is not necessary to specially provide a collecting electrode. However, since conductive oxide, such as indium tin oxide (ITO) or zinc oxide, forming the transparent electrode layer has a resistivity higher than that of metal, there is such a problem that the internal resistance of the solar cell increases. Thus, a collecting electrode (metal electrode as an auxiliary electrode) is provided on the surface of the transparent electrode layer to improve current extraction efficiency.
The collecting electrode of the solar cell is generally formed by pattern-printing a silver paste by a screen printing method. This method is simple in terms of the process itself, but has such a problem that the material cost of silver is high, and that the resistivity of the collecting electrode increases because a silver paste material containing a resin is used. For decreasing the resistivity of the collecting electrode formed of a silver paste, it is necessary to thickly print the silver paste. However, since the line width of the electrode increases with the increase of the print thickness, thinning of the electrode is difficult, and the shading loss by the collecting electrode increases.
For solving these problems, a method is known in which a collecting electrode is formed by a plating method excellent in terms of material and process costs. For example, Patent Documents 1 to 3 disclose a solar cell method in which a metallic layer made of copper or the like is formed by a plating method on a transparent electrode that forms a photoelectric conversion section. In this method, first, a resist material layer (insulating layer) having an opening section matching the shape of a collecting electrode is formed on the transparent electrode layer of the photoelectric conversion section, and a metallic layer is formed at the resist opening section of the transparent electrode layer by electroplating. Thereafter, the resist is removed to form a collecting electrode having a predetermined shape.
Patent Document 3 discloses that the line width of a plating electrode is made equal to or less than that of an under-layer electrode by forming the plating electrode layer using a mask after a formation of the under-layer electrode. In addition, Patent Document 3 discloses that a plating solution deposited on a substrate is washed off by water after a plating step, an organic solvent or the like, in view of the problem that solar cell characteristics are degraded if the solar cell, on which a plating solution remains, is exposed under a high-temperature and high-humidity environment.
Non-Patent Document 1 discloses a method in which a metallic pattern is formed by using a self-assembled monolayer as a very thin resist, and describes an electroplating method as one of the methods for forming a metallic pattern.
Patent Document 4 proposes a method in which a passivation layer (insulating layer) made of a polymer resin is formed on a transparent electrode layer, and a collecting electrode composed of an under-layer electrode made of an electroconductive paste and a metallic layer is formed thereon, wherein the metallic layer is formed on the under-layer electrode by electroplating. In this method, the passivation layer is partially dissolved by a solvent and a monomer component contained in the paste during formation of the under-layer electrode, so that an electrical contact is obtained between the transparent electrode and the collecting electrode. Since the under-layer electrode is formed on the insulating layer, shunts and short circuits due to contact of a defect portion of a semiconductor layer with the under-layer electrode are prevented.
As another method for forming a collecting electrode, Non-Patent Document 2 proposes a method in which an insulating layer made of silicon nitride or the like is formed on the surface of a crystalline silicon-based solar cell, and thereafter a silver paste is pattern-printed by a screen printing method and fired at a high temperature. In this method, since the silver paste is fired at a high temperature, the insulating layer is melted, so that electrical contact is obtained between silver particles in the silver paste and crystalline silicon.